1. Field of the Invention
The present invention relates to thin film deposition monitoring and analysis of thin films, and in particular, to apparatus, systems, devices, and methods for monitoring the deposition of ultra-thin films and nanomaterials, and analyzing the properties thereof using surface acoustic wave technology.
2. Description of Related Art
Thin films are deposited by many means, including but not limited to evaporation (thermal or e-beam), sputtering (DC or RF), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), laser assisted deposition, atomic layer deposition (ALD), and others. The most appropriate technique with which to deposit a film depends on characteristics of the particular film, including composition, electrical conductivity, thermal behavior, physical properties (such as melting point), etc. Films can be relatively thick (microns), or as thin as an atomic layer, can be continuous or discontinuous, and can have properties characteristic of the bulk material deposited, or can have widely different properties, including but not limited to nanostructures and nanomaterial based properties. In almost all cases, the thickness and other properties of the film such as conductivity, viscoelastic properties, and others are important to the ultimate function of the film. Having a real-time deposition monitor capable of monitoring deposition rate, and monitoring properties of the films during deposition and after deposition is complete, is beneficial for process control and optimization of such deposition processes. This has been recognized for decades, and much literature exists related to different ways to implement thing film deposition monitors, although historically this work has focused on “thin” films in the thickness range of microns. It is only relatively recently that the need has emerged for monitoring ultra-thin and nanostructured material deposition.
Widespread commercial application of nanomaterials has been hindered by a lack of measurement techniques capable of providing information on material quality and uniformity for use in manufacturing process control. This is true for a range of promising nanomaterials, from nanocluster metal films to molecularly imprinted polymers, and especially carbon nanotubes. Carbon nanotubes have unique thermal, electrical, and physical properties that make them well suited to many applications, including conductive and high-strength composites, nanometer sized electronics, energy storage, energy conversion, hydrogen storage, and field emission devices such as flat panel displays. R. H. Baughman, A. A. Zakhidov, W. A. de Heer, “Carbon Nanotubes—the Route Toward Applications”, Science, Vol 297, 2 Aug. 2002, 787-792. While specific commercial applications have been realized, particularly for multiwalled nanotubes (MWNTs) which have been used as electrically conductive additives in polymers and plastics since the early 1990s, manufacturing issues such as polydispersity in nanotube type, impurities, and high cost for high purity single wall nanotubes (SWNTs) limit the widespread adoption of this potentially beneficial technology.
The commercial potential for deposition monitor devices that are the subject of embodiments of the invention is therefore related to the commercial potential for nanomaterials, the most promising of which are single walled and multiwalled carbon nanotubes. A brief description of selected current and potential applications is included below.
Multiwalled carbon nanotubes have been commercially utilized in significant quantities as electrically conductive additives to plastics and polymers since the early 1990s, an application that is tolerant of MWNTs with relatively high defect densities. R. H. Baughman, A. A. Zakhidov, W. A. de Heer, “Carbon Nanotubes—the Route Toward Applications”, Science, Vol 297, 2 Aug. 2002, 787-792. Commercial automotive gas lines and filters use fillers with conductive MWNTs that dissipate the static charge buildup that could lead to explosions. SWNTs and MWNTs also appear promising as additives to plastics for enhanced strength and thermal conductivity. The high electrochemically accessible surface area of CNT arrays combined with their electrical conduction properties make them promising materials for use in high capacity “supercapacitors”, which could find widespread use in hybrid vehicles. R. H. Baughman, A. A. Zakhidov, W. A. de Heer, “Carbon Nanotubes—the Route Toward Applications”, Science, Vol 297, 2 Aug. 2002, 787-792. Nanotube actuators operate at voltages one to two orders of magnitude lower than piezoelectric or electrostrictive actuators, making them promising for low power applications (and reduction in power consumption for any application). R. H. Baughman, A. A. Zakhidov, W. A. de Heer, “Carbon Nanotubes—the Route Toward Applications”, Science, Vol 297, 2 Aug. 2002, 787-792. Field emission devices are another very promising CNT application, particularly for flat panel displays, an area actively being developed by industry. The commercial competitiveness of these displays, which can potentially have higher brightness, lower power consumption, faster response rate, and wider viewing angle than other current display technologies, will depend on the successful development of electronic technologies needed for addressing, and on concurrent advances in competitive display technologies such as LCD and non-CNT LED displays. R. H. Baughman, A. A. Zakhidov, W. A. de Heer, “Carbon Nanotubes—the Route Toward Applications”, Science, Vol 297, 2 Aug. 2002, 787-792.
Silicon based electronics have become smaller and smaller over the past few decades, producing faster processors with increased computational power. This size reduction process, however, is fundamentally limited by the physical and electrical properties of the materials being used. Carbon nanotubes are extremely promising for use in nanometer-sized electronics, allowing smaller devices with better thermal (power handling) properties and faster computational speeds. IBM has been a pioneer in developing these devices, demonstrating arrays of CNT based transistors in 2001 (Science, Vol 292, Issue 5517, Apr. 27, 2001), and logic devices with CNTs shortly thereafter (Nanoletters, Aug. 26, 2001 Web edition).
Hydrogen storage in nanotubes is another potential use, although their promise is not as clear for this as it is for other applications. R. H. Baughman, A. A. Zakhidov, W. A. de Heer, “Carbon Nanotubes—the Route Toward Applications”, Science, Vol 297, 2 Aug. 2002, 787-792.
Widespread adoption of CNTs for these and other applications has been hindered by manufacturing issues, including difficulty in obtaining homogeneous samples of one type of CNT, and impurities resulting from the manufacturing process. These issues are more significant for applications that will not tolerate high defect densities, such as nanoscale electronics, and result in higher costs for high purity NT materials. While costs have dropped greatly in the past decade, further cost reductions, particularly for high purity SWNTs, will be needed for adoption in certain commercial applications. The deposition monitor technology that is the subject of embodiments of the present invention will provide a manufacturing process monitoring and quality control tool, allowing for thermal decomposition studies of CNT batch samples, evaluation of CNT physical and electrical properties, and potentially real-time monitoring of CNT formation within growth systems. Such a tool would enhance CNT manufacturing capabilities, reducing product costs and accelerating adoption of these promising materials in a plethora of applications.
In order to provide an understanding of the aspects of the devices and systems described in embodiments of the present invention, it is necessary to first discuss conventional film deposition monitors, and specifically quartz crystal microbalance (QCM) technology and its drawbacks related to the use of QCM devices to monitoring deposition of ultra-thin films.
Deposition Monitors:
Conventional film deposition monitors include acoustic, optical, and impedance-based techniques. Acoustic techniques include QCM, thin film bulk acoustic resonator (FBAR), and SAW approaches. Each of these areas is discussed below. Optical techniques include optical absorption or attenuation, optical reflection, and optical fiber techniques. Impedance-based techniques include measurement of film conductivity based on step coverage, and others.
QCM Deposition Monitors:
Quartz crystal microbalance (QCM) devices rely on a thickness shear resonant condition in a bulk crystal (usually AT-quartz), the frequency of which is changed by the deposition of materials on the device surface. These devices were first investigated almost 50 years ago, and it is well known that for rigidly adhered acoustically thin films, the change (reduction) in frequency of resonance caused by addition of the film is proportional to the added film mass multiplied by the square of the resonant frequency of the QCM (M. Thompson and D. C. Stone, Surface-Launched Acoustic Wave Sensors, John Wiley & Sons Inc., NY, 1997), while the low electromechanical coupling coefficient of quartz means that film electrical properties do not impact device response. This simple relationship has made the QCM a very useful tool in the manufacturing of thin films of many kinds over the past few decades. Metal, dielectric, and other film deposition systems used in the manufacture of electronic components generally utilize QCM film thickness monitors to ensure proper film deposition rates and thicknesses. Systems such as electron beam deposition, sputtering, and chemical vapor deposition can all make use of this technology to monitor film deposition conditions. Typical commercial deposition monitors claim resolutions that are on the order of 1-2 angstroms for film thicknesses of 1000 angstroms or less. However, deposition of ultrathin films (under 50 angstroms thick) and nanostructured materials present particular challenges, and QCM devices are of limited usefulness for monitoring the deposition of these materials. The sensitivity of the QCM to thermal transients when the shutter is opened in an e-beam system, for example, may result in slight errors in the reported deposition rate and film thickness. For nanostructured films, such errors can result in film morphologies that differ widely from the desired films. In addition, deposition of multiple films can easily push the device into a regime of lower sensitivity as the overall film thickness increases. In addition to monitoring film deposition, QCM devices have (in the past) achieved widespread use as a method for monitoring molecular contamination of surfaces in manufacturing and clean room applications (these applications have since transitioned to surface acoustic wave (SAW) monitors due to enhanced sensitivities).
Recently, QCM devices have been used as a tool to characterize nanomaterials such as carbon nanotubes, and for molecular and biological imprinting and sensing research. Drop-casting of solutions containing carbon nanotubes onto QCMs has been studied by researchers at the University of Maryland Baltimore County (UMBC) and the National Institute of Standards and Technology (NIST). S. Hooker, A. Kar, and R. Schilt, “Evaluation of Quartz Crystal Microbalance Thermal Analysis in Characterization of Carbon Nanotube Purity”, Poster session, 3rd NASA-NIST Workshop on Nanotube Measurements, September 26-28, NIST, Gaithersburg Md.; H. Wilson, S. Hooker, and A. Kar, “Purity of Carbon Nanotubes: Separating the Nanotube from the Dispersant in Thermal Decomposition Profiles”, Poster session, 3rd NASA-NIST Workshop on Nanotube Measurements, September 26-28, NIST, Gaithersburg Md. These studies have shown promising results for using the QCM devices to evaluate sample homogeneity, and to study the thermal decomposition properties of the CNT samples. Molecular imprinting has been studied as a means to produce sensor materials selective for both molecular analytes and biological moieties such as cells and microorganisms. QCM devices have been useful tools for measurement of such sensor films. P. A. Lieberzeit, G. Glanzing, M. Jenik, S. Gazda-Miarecka, F. L. Dickert, and A. Leidl, “Softlithography in Chemical Sensing—Analytes from Molecules to Cells”, Sensors 2005, 5, 509-518; M. Penza et. al, “Carbon nanotubes-coated multi-transducing sensors for VOCs detection”, Sensors and Actuators B: Chemical, Vol. 111-112, 11 Nov. 2005, 171-180; H. Chen et. al., “The application of CNT/Nafion composite material to low humidity sensing measurement”, Sensors and Actuators B: Chemical, Vol. 104, 3 Jan. 2005, 80-84; M. Consales et. al., “Carbon nanotubes thin films fiber optic and acoustic VOCs sensors: Performance analysis”, Sensors and Actuators B: Chemical, Vol. 118, 25 Oct. 2006, 232-242. Dissipation monitoring of QCM devices (QCM-D) utilizes these devices in a dynamic mode—the drive energy to the QCM is switched off, and the decay of the damped oscillation is measured. This technique can be used to provide additional information about adhered layers, and has been applied primarily in polymeric and biological research. A. Jaiswal, “In-Situ Characterization of Polymer Layer Formation by Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D)”, Nanotech 2007 Conference.
The QCM devices discussed generally operate in the low MHz range—often 5 MHz to 15 MHz. These devices are limited to low frequency operation because the crystal substrate thickness determines the operating frequency, and higher frequencies would require very thin, fragile crystals. Theoretical mass detection limits of typical QCM devices are generally in the 10−12 gram (picogram) range. M. Thompson and D. C. Stone, Surface-Launched Acoustic Wave Sensors, John Wiley & Sons Inc., NY, 1997.
QCM TGA Monitors:
Recently, quartz crystal microbalance (QCM) devices have been utilized as a measurement platform to perform microscale thermogravimetric analysis (TGA) of nanoparticles, thin films, and other materials. Mansfield, E. et. al., “Quartz Crystal Microbalances for Microscale Thermogravimetric Analysis”, Anal. Chem. 2010, 82, 9977-9982; Mansfield, E. et. al., “Applications of TGA in quality control of SWCNTs”, Anal Bioanal Chem (2010) 396:1071-1077. This approach has the benefit of requiring only micrograms of sample, compared to several milligrams of sample required by conventional TGA techniques. This work has shown that QCM-based TGA can provide a 3-fold increase in resolution compared to the state-of-the-art using standard TGA techniques. Mansfield, E. et. al., “Quartz Crystal Microbalances for Microscale Thermogravimetric Analysis”, Anal. Chem. 2010, 82, 9977-9982. However, the cited work performed measurements in a batch type operation, where QCM crystals with samples were measured, removed from their fixtures for heating to a pre-selected temperature and cooling, and then re-inserted into their fixtures for subsequent measurement of the QCM.
Embodiments of the present invention provide significant improvements over this type of QCM, in that real-time in-situ measurements can be made as temperature varies to provide immediate feedback on reaction kinetics and sample properties as the sample is heated and cooled.
FBAR Deposition Monitors:
Film bulk acoustic resonators (FBARs) are acoustic resonant devices produced by layering materials with differing acoustic properties to form an acoustic cavity structure suitable to sustain resonance at a particular frequency. FBAR devices have been used by Larson as film deposition monitors. Larson, J. D., et. al., “Systems and methods of monitoring thin film deposition”, U.S. Pat. No. 6,668,618, 2003. In this example, as in the SAW application discussed below and in Hemphill, R. B., “A surface wave thickness monitor for thin evaporated films”, Proceedings of the IEEE International Ultrasonics Symposium, p. 525-529, 1984, a reference acoustic device of the same type (FBAR or SAW) that is shielded from deposition of the film being deposited is used to provide a reference response during deposition. This reference device can be thermally coupled to the acoustic device exposed to film deposition, allowing unambiguous separation of film thickness and thermal effects.
SAW Deposition Monitors:
Since 1979, surface acoustic wave (SAW) devices have been recognized as a promising alternative to QCMs for sensing applications requiring higher sensitivity and lower detection limits. H. Wohltjen and R. Dessey, Anal. Chem., 1979, 51, 1458, 1465. The ability of SAW devices to perform as an analytical tool to evaluate changes in the properties of deposited films was also recognized some time ago, as in Wohltjen's work on thin film polymer characterization. H. Wohltjen and R. Dessey, Anal. Chem., 1979, 51, 1470. Due to construction of SAW devices, the operating frequency can be set independently of the crystal substrate thickness, by patterning interdigital electrodes on the crystal surface with selected pitch. Typical SAW device operating frequencies range from 50 MHz to 3 GHz. Since the mass sensitivity of these devices is (to first order) proportional to the square of the operating frequency, SAW devices have substantially higher mass sensitivity. In 1984, Wohltjen argued that SAW devices at 3 GHz with active surface areas of 10−4 cm2 would have a realistic detection limit of roughly 3×10−15 grams (femtograms), taking into consideration typical measurement system noise. H. Wohltjen, Sens. Actuators, 1984, 5, 307. Thus, SAW microbalances are capable of two to three orders of magnitude greater mass sensitivity than QCM devices. Also in 1984, Hemphill published his work on a SAW thickness monitor for thin evaporated films. Hemphill, R. B., “A surface wave thickness monitor for thin evaporated films”, Proceedings of the IEEE International Ultrasonics Symposium, p. 525-529, 1984. This approach utilized an evaporated shorting layer on the surface of the SAW propagation path to short out the electric field at the surface of the device. This made the monitor insensitive to changes in conductivity of the film being deposited, making it sensitive only to the mass and viscoelastic properties of the film. While useful for certain films, this prevented use of film conductivity as a parameter for deposition monitoring. This work (Hemphill, R. B., “A surface wave thickness monitor for thin evaporated films”, Proceedings of the IEEE International Ultrasonics Symposium, p. 525-529, 1984) also discusses the use of this device to measure relatively thick films, with thicknesses of up to 100 microns or more.
Molecular contamination monitors adopted SAW microbalances as an improvement over QCM devices several years ago. “Monitoring Molecular Contamination of Critical Surfaces in Semocinductor Manufacturing”, Application Note, Particle Measurement Systems, 2002, www.pmeasuring.com; D. Rodier, “Method and apparatus for monitoring molecular contamination of critical surfaces”, U.S. Pat. No. 6,945,090, 2005, the disclosure of which is herein incorporated by reference in its entirety. These contamination monitors measure the accumulation of contamination on critical surfaces such as silicon dioxide, metals, and polymers relevant to the semiconductor manufacturing clean room environment. Carbon nanotubes have also been used on SAW devices as chemically selective elements for vapor sensing (M. Penza, F. Antolini, and M. V. Antisari, “Carbon nanotubes as SAW chemical sensors materials”, Sensors and Actuators B: Chemical, Vol. 100, 1 Jun. 2004, 47-59; M. Penza, F. Antolini, and M. V. Antisari, “Carbon nanotubes-based surface acoustic waves oscillating sensor for vapor detection”, Thin Solid Films, Vol. 472, Issues 1-2, 24 Jan. 2005, 246-252), as have numerous metal oxide and polymer films (M. Thompson and D. C. Stone, Surface-Launched Acoustic Wave Sensors, John Wiley & Sons Inc., NY, 1997). In addition to being highly mass sensitive, with acoustic wave velocity changing with deposited mass on the surface, SAW devices respond to changes in other parameters of coating films and fluids in contact with the device surface. Film conductivity will change the SAW propagation velocity and attenuation if a substrate such as lithium niobate (which has a high electromechanical coupling coefficient) is used. Physical film or fluid properties, such as stiffness, elasticity, viscosity, etc. also alter device performance measurably, producing changes in velocity and/or attenuation. Various surface launched acoustic wave propagation modes can be used to generate surface wave devices that will operate in either liquid or gaseous media, and the device can be optimized for the anticipated environment. Finally, SAW devices are extremely rugged and can operate over a very wide temperature range. With colleagues, the inventor is currently developing sensors for NASA applications that require operation at temperatures ranging from cryogenic to almost 1000° C. These devices have thus far been demonstrated to tolerate repeated cycling from cryogenic (liquid nitrogen) temperatures to room temperature, and from room temperature to temperatures in excess of 300° C. This operating temperature range has been demonstrated successfully using both single crystal quartz and lithium niobate substrates. The upper limit on device testing has been limited by our test capability. Fundamentally, the upper limit on operating temperature for quartz devices is limited (to no more than 500-550° C.) by the crystalline α-β phase transition that occurs at 573° C., although traditional quartz devices operate near ambient room temperatures in order to take advantage of the zero first order temperature coefficient of frequency near room temperature. Lithium niobate devices can operate at higher temperatures than quartz, and devices fabricated on langasite, langanite, or langatate can operate up to 1000° C. or more.